This invention relates to a measuring oscillator for temperature sensing, which produces a signal having a temperature-dependent frequency, and comprises an amplifier fed back positively through an all-pass filter, the all-pass filter comprising a bipolar phase-determining circuit element having a temperature-dependent ohmic resistance value which varies the frequency to be measured of the oscillator signal in dependence upon temperature.
The magazine etz-b, Vol. 25 of 1973, No. 9, p. 220-222 discloses a measuring oscillator of the kind described in the opening paragraph having an all-pass filter which comprises, as a phase-determining ohmic resistive element, a platinum resistance thermometer. At temperatures between 0.degree. C. and 100.degree. C., oscillator frequencies between 800 Hz and 1 kHz are obtained. As a result of this comparatively small frequency variation of 20% for a temperature variation of 100.degree. C. small temperature variations can be measured with insufficient accuracy. The small frequency variations then obtained require a frequency measuring appartus of high resolution.